Planet Hunting: Finding Earth-like Planets

The University Observatory Jena with its 90-cm telescope in Grossschwabhausen near Jena, where three of the six new transits of WASP-3b were observed

A team of astronomers from Germany, Bulgaria and Poland has used a completely new technique to find an exotic extrasolar planet. The same approach is sensitive enough to find planets as small as the Earth in orbit around other stars. The group used transit timing variation (TTV) to detect a planet with 15 times the mass of the Earth in the system WASP-3, 700 light years from the Sun in the constellation of Lyra.

TTV was suggested as a new technique for discovering planets a few years ago. Transits take place where a planet moves in front of the star it orbits, temporarily blocking some of the light from the star. So far, this method has been used to detect a number of planets and is being deployed by the Kepler and Corot space missions in its search for planets similar to the Earth.

If a (typically large) planet is found, then the gravity of additional smaller planets will tug on the larger object, causing deviations in the regular cycle of transits. The TTV technique compares the deviations with predictions made by extensive computer-based calculations, allowing astronomers to deduce the makeup of the planetary system.

Led by post-doc Gracjan Maciejewski of Jena University in Germany, the research is based on the collaborative effort of scientists at the Astrophysical Institute of the Friedrich Schiller University in Jena, Germany, the Institute of Astronomy of the Bulgarian Academy of Sciences in Sofia, Bulgaria, and the Torun Centre for Astronomy of the Nicolaus Copernicus University in Torun, Poland.

For this search, the team used the 90-cm telescopes of the University Observatory Jena and the 60-cm telescope of the Rohzen National Astronomical Observatory in Bulgaria to study transits of WASP-3b, a large planet with 630 times the mass of the Earth.

“We detected periodic variations in the transit timing of WASP-3b. These variations can be explained by an additional planet in the system, with a mass of 15 Earth-mass (i.e. one Uranus mass) and a period of 3.75 days”, said Maciejewski. “In line with international rules, we called this new planet WASP-3c.”

This newly discovered planet is among the least massive planets known to date and also the least massive planet known orbiting a star which is more massive than our Sun.

This is the first time that a new extra-solar planet has been discovered using this method. The new TTV approach is an indirect detection technique, like the previously successful transit method.

The discovery of the second, 15 Earth-mass planet makes the WASP-3 system very intriguing. The new planet appears to be trapped in an external orbit, twice as long as the orbit of the more massive planet. Such a configuration is probably a result of the early evolution of the system.

The TTV method is very attractive, because it is particularly sensitive to small perturbing planets, even down to the mass of the Earth. For example, an Earth-mass planet will pull on a typical gas giant planet orbiting close to its star and cause deviations in the timing of the larger objects’ transits of up to one minute.

This is a big enough effect to be detected with relatively small one-meter diameter telescopes, and discoveries can be followed up with larger instruments. The team is now using the 10-meter Hobby-Eberly Telescope in Texas to study WASP-3c in more detail.